1. Field of the Invention
The invention pertains to stable aqueous emulsions of epoxy- and organo-substituted branched organopolysiloxanes which retain their epoxy functionality upon storage.
2. Description of the Related Art
Epoxy-functional carbon-based polymers are widely used. In the coatings field in particular, solvent-based epoxy resins have been used to provide durable coatings. However, organic solvents have come under exceptional scrutiny, and thus aqueous emulsions of carbon-based resins such as those based on bisphenol A have been developed. These emulsions are possible due to the relatively low molecular weight of the epoxy resins used, and their relatively high polarity which assists in their dispersion. However, coatings made from such epoxy resins tend to yellow over time, and are subject to UV degradation when used outdoors. In addition, they are subject to loss of gloss, and to thermal degradation as well.
Organopolysiloxanes are highly thermally stable and stable to UV light as well. It would be desirable to combine these desirable features of organopolysiloxanes with the curing possibilities offered by incorporation of epoxy groups, and indeed, epoxy-functional organopolysiloxanes such as α,ω-bis(glycidoxypropyl)-terminated polydimethylsiloxanes are commercially available. However, these are used either neat or dissolved in organic solvent. Due to the greater hydrophobicity of the organopolysiloxane backbone, preparation of aqueous emulsions is unknown. Moreover, products prepared by curing such epoxy-functional organopolysiloxanes still do not have the thermal resistance and chemical resistance desired. Increasing the molecular weight of the organopolysiloxane component increases chemical and thermal resistance, but at the cost of requiring larger amounts of solvent. The decreased epoxy equivalent weight renders the production of hard coatings very difficult, if not impossible.
Martin et al. U.S. published application US2011/0190415A1 discloses aqueous emulsions prepared by emulsifying at least one organopolysiloxane resin containing condensable hydroxyl groups, an epoxy-functionalized organopolysiloxane fluid such as an epoxycyclohexylethyl-substituted polydimethylsiloxane, and a water-soluble, aminoalkyltrihydroxysilane or hydrolysate thereof. Upon mixing, the epoxy groups of the epoxy-functional silicone fluid react with the condensable groups, and thus the final emulsion no longer contains epoxy functionality. The compositions must be free of alkoxysilanes. The dispersed particles are able to form water resistant surfaces on masonry products, by absorption into the pores of the masonry and coalescence. The low molecular weight of the starting materials allows for a fine emulsion to be prepared, which retains its dispersion characteristics after the raw materials react in situ.
In like manner, Liu et al. U.S. published application US2011/0201751A1 discloses the preparation of organopolysiloxane resin emulsions by emulsifying epoxyalkyl-functional silanes with alkoxysilanes in water in the presence of a condensation catalyst such as triethanolamine. The result is a polymer which is condensed through both the normal elimination of alcohol from the alkoxysilane groups and condensation of silanol groups as well through the reaction of epoxy groups with silanol groups. The result is a dispersion of a highly crosslinked resin containing residual, unreacted alkoxy groups, and 1,2-dihydroxypropyloxypropyl groups, but with no residual epoxy groups. In both US2011/0190415 and US2011/0201751, the epoxy groups are no longer available for reaction.
It would be desirable to provide epoxy-functional organopolysiloxanes of relatively high molecular weight, which can subsequently be crosslinked into a variety of cured products, including hard coatings, and which are thermally and chemically stable, without the use of extensive amounts of solvent. These emulsions should retain their epoxy functionality upon storage.